INTRODUCTION 43 



missible to their offspring by heredity. But both these 

 are facts that no one can call into question, as they 

 come before us every day. We see daily how brothers 

 and sisters differ from each other, and at the same 

 time how many peculiarities of the father and mother 

 are handed on to their children. These two factors 

 are the chief means by which natural selection works, 

 producing one species from another. We have seen 

 above the way in which it does this. 



Let us recall the process. Every species is found to 

 be over-productive in reproducing itself ; that is to say, 

 it brings into the world a larger progeny than there is 

 room for. A great number of these, therefore, must 

 fall victims to unfavourable conditions or to enemies, 

 and these will be — as a general rule, and apart from 

 accident — precisely the least endowed in body and 

 intelligence. In other words, those animals will survive 

 longest and reproduce most which are in every respect 

 — as to inclement weather, enemies, and so on — the best 

 able to resist. Thus the fittest among the varieties 

 that casually arise will be preserved, and their useful 

 characters will be accentuated in the course of many 

 generations, since each new generation over-produces 

 in turn, and natural selection again chooses the best 

 to survive. If the new characters are connected by 

 intermediate forms with those of the parental species, 

 which may have adapted itself to the struggle for life in 

 the original form in some sheltered locality, we may still 

 speak of varieties. If the intermediate forms have died 

 off — and that will quickly follow — it is clear that animals 

 with completely distinct characters will be left, instead of 



